This invention relates generally to optical devices, and more particularly relates to optical device nonlinearities.
An optical material can be characterized by its optical nonlinearity, which relates the nonlinear dependence of the polarization of the material to the optical field strength in the material. For example, the Kerr nonlinearity is distinguished by an index of refraction that depends linearly on the intensity of light present in the nonlinear material. Such a nonlinearity, and optical nonlinearities in general, enable a range of important optical phenomena, including frequency mixing, supercontinuum generation, and soliton propagation. Optical signal processing, such as higher-harmonic generation, and optical devices, such as optical switches, are fundamentally enabled by optical nonlinearities. For example, refractive index changes can be used to change the transmission characteristics of resonant cavities and other structures by modifying the effective optical path length of the structure or shifting the cavity resonances to alternate frequencies.
In general, optical nonlinearities are typically quite weak in an optical material. But many optical processing operations and optical devices cannot operate fully as-intended without a relatively strong nonlinearity. For example, optical bistable switching device power consumption, optical dispersion compensation strength, and two-beam coupling length all depend directly on the degree of nonlinearity of an optical material employed in a device or system. As a result, device characteristics are often adjusted in an attempt to enhance the nonlinearity of the optical device. For example, the geometry of an optical device can be tailored specifically for nonlinearity enhancement of the device. But although this approach does enhance nonlinearity, it typically also increases the characteristic quality factor of the device, which in turn decreases, e.g., the switching speed of the device. In an alternative strategy, the optical device material can be specifically selected primarily for the degree of nonlinearity exhibited by the material. But in general, highly nonlinear materials are characterized by other properties that are deleterious for optical processing. For example, many highly nonlinear materials are also characterized by a degree of optical absorption that is prohibitively high for many optical processing applications. Thus, even with optical device geometry and material design that are directed specifically to nonlinearity enhancement, the performance of optical processing and optical devices is constrained by the generally low nonlinearity of optical materials.